Outline

Background: The majority of modern heavy ion radiotherapy has been performed at NIRS in Japan. For some tumor sites unparalleled local control rates have been achieved. At NIRS a biologically flat SOBP is created with an adequately shaped ridge filter. Ridge filters have been shaped for a dose per fraction of 2.7 GyE with an approach based on the Zaider Rossi (ZR) model. In clinical practice the same ridge filters have been employed even with different dose per fractions. The prescribed clinical dose has been scaled linearly with the physical dose.

Material and methods: A ridge filter can be univocally described by the physical dose profile that produces a flat clinical 2.7 GyE along the SOBP (given as function of penetration depth) together with the alfamix and betamix parameters of the ZR model along the SOBP. Employing the same ZR model we have recalculated the biological equivalent dose for different dose per fraction ranging from 2.7 to 4.4 GyE. Subsequently we have averaged this recalculated dose along the SOBP and have compared the result with the nominal prescribed dose thus deriving ridge-filter specific dose-dependent biologic correction factors. Dose reporting at NIRS is not limited to biological effectiveness; the biological equivalent dose of 1.8 Gy is reported as a clinical equivalent dose of 2.7 GyE in order to achieve the same clinical RBE of the fast neutrons previously employed at NIRS in the single point of the SOBP where dose averaged LET is similar to fast neutrons LET. The same scaling factor of 1.5 is applied to all the other points of the SOBP. There is no simple way to calculate what may happen to this scaling factor when changing dose per fraction. Starting from the physical dose and the clinical flat dose at 2.7 GyE and using the ZR model we have recalculated interpolated clinical alfamix and betamix values that would produce the same flat clinical dose. As we had only one equation but two parameters for each point, we have done two different assumptions: constant alfa/beta ratios or constant beta values. With these two different new sets of alfamix and betamix we have recalculated clinical correction factors.

Results: Mean biological correction factors (averaged on all the ridge filters) ranged from 0.96 to 0.93 and mean clinical correction factors ranged from 0.95 to 0.91 for prescribed doses of 3.6–4.4 GyE.

Conclusions: Different tumors have been successfully treated at NIRS and many dose escalation trials have been performed. In order to apply this invaluable clinical knowledge to other facilities it is necessary to calculate correction factors to account for the different way in which clinical equivalent dose is calculated and reported. The correction factors calculated in this paper are intended as a first step to eliminate a confounding element and to allow a more correct evaluation of real differences due to choice of the model and of the biological systems.